Trend 3.1
Trend 3.1 of the Boron Group (Group IIIA) Trends is that the chemistry of boron is quite different from that of the heavier Group IIIA (Boron column) elements. It differs from aluminum in the following ways. a) Its oxide and hydroxide are acidic, where as those of aluminum are amphoteric. b) Boron is a semiconductor which has various polymorphs based on icosohedral boron cages, whereas aluminum is a metal with a close packed structure. Boron is very inert and only attacked by hot concentrated oxidizing acids. c) No simple salts of B3+ are known, whereas those of Al3+ are numerous and well documented. d) Boron forms a wide range of hydrides, which have cage structures. (AlH3)4 has a polymeric structure which resembles that of AlF3. e) The stereochemistries of many boron compounds are based on trigonal sp2 and tetrahedral sp3 geometries. In the latter the octet rule is obeyed. Aluminum forms many compounds with tetrahedral, trigonal bipyramidal, and octahedral geometries. f) Multiple pπ- pπ bonding in boron-nitrogen, boron-oxygen, and boron-fluorine compounds is more significant than for the corresponding aluminum compounds. {BN}x for example adopts a graphite structure. Acidic oxide and hydroxides Boron’s oxide and hydroxide are acidic, where as those of aluminum are amphoteric. The metal/non-metal line plays a large role in the distinction between boron oxides and hydroxides and aluminum oxides and hydroxides. Aluminum lies below the metal/non-metal line and is, therefore, more metallic than Boron. The overall trend of the periodic table tells us that oxides and hydroxides of metals are more acidic while oxides and hydroxides of non-metals are more basic. Since aluminum lies just below the metal/non-metal line, its oxides and hydroxides can function as either an acid or a base depending on what other compounds they are surrounded with. Here’s an example of Aluminum oxide’s amphotericity: Al2O3 + 6 HCl → 2 AlCl3 + 3 H2O Al2O3 + 6 NaOH + 3 H2O → 2 Na3Al(OH)6 Boron will associate with OH in the following manner: H3BO3 The form of this appears as an acid and it certainly acts like one. The name of this compound is boric acid. Electronegativity Looking at these types of compounds in greater detail, the real deciding factor between how the compounds interact is based on electronegativity. Boron has an electronegativity of 2, while Aluminum has an electronegativity of 1.5. Oxygen and Hydroxide have electronegativities of 3.5 and 2.8, respectively. Since Boron’s electronegativity is much closer to oxygen and hydroxide, it will interact much more covalently with them than Aluminum will. Boron oxides and hydroxides are more prone to electron sharing while aluminum’s are slightly more prone to an ionic type interaction than Boron. The covalency of the B-O and B-OH interactions allows the O and OH to act more acidically, while the slightly more ionic Al-O and Al-OH interactions allow the O and OH in these compounds to act either way, depending on the surrounding compounds. Boron is a semiconductor with polymorphs while Aluminum is a metal with close packed structure No simple salts of B3+ are known, whereas those of Al3+ are numerous and well documented Boron forms a wide range of hydrides, which have cage structures. (AlH3)4 has a polymeric structure which resembles that of AlF3 To begin, the bonding of these cages must be explained. Bonds most commonly consist of the sharing of two electrons between two atoms. Cluster bonding is similar, except the electron cloud is shared evenly over several atoms and bonds are not clearly defined between any two atoms. Borohydride cages are common because B-H and B-B bonds are nonpolar covalent bonds, so electrons can move freely. Carboranes are also possible because C-B and C-H bonds are also nonpolar covalent, so the resulting structure is also stable. Cluster bonding is electron-deficient, so electrons are more creatively shared throughout the structure than they would normally be. As a result, the borohydride cages often have negative charges to stabilize the structures. Another aspect of this electron deficiency is that vertices are occasionally “missing”, so the cage forms an open structure rather than the “normal” closed cage structure. In order to accurately predict and describe these odd cage structures, chemists use Wade-Mingos rules to name the cages based upon electrons involved in cluster bonding. The rules are as follows: : 1. Count the number of B-H bonds and the corresponding electron values. It is assumed that of the total valence electrons of boron and hydrogen, two electrons contribute to cluster bonding (two out of three of boron’s valence electrons) while the other two electrons from hydrogen and boron are used to stabilize the B-H bond. So, the number of borohydride bonds multiplied by two equals the total number of electrons from B-H bonds. : 2. Count any extra hydrogen atoms that are part of the structure, but are not involved in B-H bonds. These each contribute one electron to the total of cluster-bonding electrons. : 3. Add up the values calculated above in addition to any overall charge that the cage might have. Then, to figure out the structure type and the correct prefixes for the borohydride cages, see the table below. : 4. If the structure should happen to be a carborane (the cage contains some C-H bonds instead of only B-H), the above rules do still function! However, in this case, the C-H bond contributes three electrons to the cluster bonding rather than the two listed above for B-H bonds. Finally, aluminum hydrides do not form these strange cage strucutres, but instead they form polymers like (AlH3)4. This occurs because aluminum is larger and has more metallic tendencies than boron which is a nonmetal. Metallic tendencies mean that aluminum bonds less covalently (molecularly) and it moves towards creating more lattice-like structures. Repeated polymers are an example of a transition state between the molecular form of most nonmetals and the rigid ionic lattice of most metallic hydrides. The Diagonal Effect is another reason for the unique Al-H polymers because Al bonds like Be, which also tends to form polymeric structures rather than ionic lattices. The stereochemistries of many boron compounds are based on trigonal sp2 and tetrahedral sp3 geometries. Aluminum forms many compounds with tetrahedral, trigonal bipyramidal, and octahedral geometries Multiple pπ- pπ bonding in boron-nitrogen, boron-oxygen, and boron-fluorine compounds is more significant than for the corresponding aluminum compounds {BN}x for example adopts a graphite structure.